Photoresist, photolithography method using the same, and method for producing photoresist

ABSTRACT

There is provided a positive photoresist for near-field exposure excellent in light utilization efficiency even with small layer thickness of the photoresist layer for image formation, and allowing for reduced pattern edge roughness, and a photolithography method including a step of exposing by the near-field exposure the photoresist layer for image formation made thereof. In a positive photoresist containing an alkali-soluble novolak resin and a quinone diazide compound, the film thickness of the photoresist at the time of exposure is not larger than 100 nm, and the absorption coefficient of the photoresist α (μm −1 ) for the exposure light is such that 0.5≦α≦7.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a photoresist which is used forphotolithography to form patterns inclusive of fine patterns equal to orsmaller than in size the light wavelengths of the exposure light, andthe photolithography using thereof.

[0003] 2. Related Background Art

[0004] Recently, it has become indispensable to make photolithographyfurther finer, with the developments of large capacity semiconductormemories and speed-enhancement and large-scale integration of CPUprocessors.

[0005] The lights used in photolithography apparatuses have continued tobecome shorter in wavelengths as means for making photolithographyfiner, and at present near ultraviolet laser lights are used and henceit is possible to make microfabrication of the order of 0.1 μm.

[0006] In order to make photolithography much finer, however, there aremany problems to be solved, including the further shortening of laserwavelengths, development of lenses usable in such wavelength regions,miniaturization of equipments, etc.

[0007] On the other hand, methods which use near-field light have beenproposed for the purpose of optical manufacturing of photoresistpatterns with widths not larger than the wavelengths of the lights used.

[0008] For example, Japanese Patent Application Laid-Open No. 7-106229discloses a method for near-field exposure based on a probe scanningwhich uses a probe made by sharpening a tip of an optical fiber by wetetching.

[0009] In addition, for the purpose of solving the problem that theabove-mentioned method is slow in throughput, many proposals such asJapanese Patent Application Laid-Open No. 11-145051 are made on the enbloc near-field exposure with photomasks.

[0010] The merits provided by near-field exposure are that the minimumfabricable pattern width is independent of the wavelength of a lightused, but is determined by the aperture of the probe and photomask used.Thus, if a semiconductor laser, for example, is used as a light sourcefor exposure, there is a merit that the apparatus can be made smallerowing to the extremely reduced size of the light source, which alsoreduces the unit cost of an exposure apparatus.

[0011] Accordingly, since the exposure sensitivity of the photoresist iseffective in the exposure light wavelengths of about 200 to 500 nm, ablue semiconductor laser can be used as an exposure light source withinthis range to make the apparatus compact. Alternatively, ageneral-purpose mercury-arc lamp can be used to provide the exposurelight of a high output power, where it is preferable to use the g-lineand i-line photoresists in consideration of photoresist sensitivity.

[0012] Furthermore, the g-line and i-line photoresists are at presentused as general-purpose materials, large in variety, easily available,and inexpensive, and hence there is a merit that the degree of freedomfor process is high and the cost can be reduced. Since in the near-fieldexposure, the width magnitude of the fabricable pattern is not limitedby the light wavelength used, there is a possibility that themicrofabrication can be made with the g-line and i-line photoresists.

[0013] As the g-line and i-line photoresists, the alkali-soluble novolakresins containing a compound comprising the naphthoquinone diazide groupas a photosensitive agent have long been used. In Japanese PatentApplication Laid-Open No. 7-319157, an example for the above-mentionedphotoresist is disclosed, where a high etching-selectivity is shown tobe generated in the patterns obtained by the i-line exposure. Sincecommercially available g-line or i-line photoresists, inclusive of theabove-mentioned case, are supposed to be used in the conventionalphotolithography that employs steppers and aligners, that is, in amethod that employs for exposure a light passing through apertures of aphotomask but not in the near-field exposure, the minimum pattern widthcorresponds to the resolution of the order of several hundreds nm toseveral μm, and the photoresist-film thickness is usually set to beabout 0.5 to 1 μm or larger. For example, in Japanese Patent ApplicationLaid-Open No. 7-319157, a case of the film thickness of 1.5 μm isdisclosed.

[0014] In the conventional photolithography methods, mercury-arc lampsand excimer lasers are used as exposure light sources, so that theexposure intensities fall in the range of several tens to severalhundreds mJ/cm². The photoresists used are required to have sufficientsensitivities for these exposure intensities, and to have such filmthicknesses that they can tolerate etching in substrate processingsubsequent to pattern formation.

[0015] In the near-field exposure method, however, the photoresist-filmthickness cannot be as thick as those in the conventional methods. Thereasons for this will be described below.

[0016] In the near-field exposure, since a photoresist is exposed to thescattered light produced by disturbing the near-field light with thephotoresist, there is observed a tendency that the large thickness ofthe image-forming photoresist layer results in the large widths of theformed patterns. This is illustrated in FIGS. 4A and 4B, where referencenumeral 204 denotes a mask base and 205 denotes a light shielding film.

[0017] By making the exposure light 505 stream into the photomask havingmicroapertures 513, the near-field light 510 is formed in theneighborhood of a microaperture 513 (FIG. 4A). When the photomask andthe photoresist 503 are brought closer together (FIG. 4B), thenear-field light 510 is scattered by the photoresist 503 placed on thesubstrate 504, the reacted photoresist portion 501 is then formed in thephotoresist 503. When the photoresist film is thick, the extension ofthe reacted photoresist portion toward the substrate 504 is enhanced,resulting in the broadening of the fabricable pattern widths. When theintervals of the microapertures are small, the reacted photoresistportions resulting from these apertures overlap each other, providing amuch broader line width of the formed pattern. Accordingly, anembodiment with large photoresist-film thickness can not make the bestuse of the merit of the near-field exposure. In order to take advantageof the merit of the near-field exposure, the film thickness of thephotoresist is desirably smaller than the mask aperture diameter whichprovides near-field light.

[0018] Since the lithography using near-field light aims at suchmicropattern formation that cannot be obtained by the conventionalmethods, in general the smallest dimension of the mask aperture is notmore than 100 nm. Accordingly, the film thickness of the photoresistshould be not more than 100 nm.

[0019] With such a small film thickness, however, pattern shapes afterthe exposure and development of the photoresist tend to be nonuniform.In other words, the edges of the patterns do not follow the prescribedlines or curves but have irregularities. The irregularities, that is,the pattern edge roughness is due to the photoresist remaining asaggregates of the order of 10 μm in diameter after development. Theyadversely affect the dimensional accuracy in the patterns finer than 100nm to cause problems.

[0020] The present inventors used the above-mentioned commercial g-linepositive photoresist to conduct the near-field exposure to make thepatterns of 200 nm in pitch and 70 nm in line width with the exposurelight at 442 nm wavelength, and observed the sectional shapes by a SEMto find that the pattern edge roughness was large and in addition therectangularity was poor, which rectangularity will be explained below.

[0021] In general, it can be said that the magnitude of the pattern edgeroughness acceptable in device fabrication is 10% of the pattern width,while there occurs fierce pattern edge roughness in the patterns made asmentioned above, possibly giving rise to adverse consequences in devicefabrication.

[0022] By the way, the pattern edge roughness concerned is defined interms of dispersion of the widths of the fabricated patterns as follows.

(Pattern edge roughness)={(maximum width for the fabricatedpatterns)−(minimum width for the fabricated patterns)}/(assumed patternwidth)

[0023] For instance, when the widths of the patterns fabricated with theassumed pattern width of 1 μm spread from 0.9 μm to 1.1 μm indifference, the pattern edge roughness amounts to

(1.1−0.9)/1=0.2

[0024] From the above equation, the pertinent pattern edge roughness isfound to be 20%. The pattern edge roughness was 50% for theabove-mentioned fabrication of the patterns having a line width of 70nm.

[0025] In addition, in the present proposal, as a method for numericallyrepresenting the precision of the fabricated patterns, the“rectangularity” quantity is defined as follows:

(Rectangularity)={(assumed pattern width)−(magnitude of the “sheardroop” in a fabricated pattern)}/(assumed pattern width)

[0026] For instance, for such patterns as shown in FIG. 6, the assumedpattern width 602 is 100 nm, the magnitude 601 of the shear droop of thefabricated patterns is 20 nm, resulting in a rectangularity of 0.8, thatis, 80%. In FIG. 6, reference numeral 103 denotes a photoresist and 104denotes a substrate.

[0027] The rectangularity was found to be 50% for the above-mentionedfabrication of the patterns having the line width of 70 nm.

[0028] In the present proposal, the object is to fabricate patterns withthe rectangularities not lower than 80%. With the rectangularity lowerthan 80%, the subsequent process tolerance is diminished, resulting inthe throughput lowering and cost rising with a high degree oflikelihood.

[0029] Efficiency for light utilization in the near-field exposure willbe explained below.

[0030] The absorption coefficient α (μm⁻¹) of the photoresist measuredby the present inventors with a laser of the 442 nm wavelength was 0.08,which photoresist is a g-line positive photoresist commerciallyavailable and assumed to be used in the exposure processing of a film ofabout 1 μm in thickness and the minimum pattern width of about 450 nm,by use of a stepper and an aligner.

[0031] Accordingly, when the photoresist is applied to the substrate soas to make a film of 1 μm in thickness, the transmittance of theresulting film is 92%. When the same photoresist is applied to thesubstrate so as to make a film of 100 nm in thickness, the transmittanceof the resulting film is 99%.

[0032] A large amount of transmitted light means that most part of thelight transmits through the photoresist layer without being absorbed,resulting in boosting the possibilities that the rectangularity isdeteriorated due to the perturbation of the pattern side wall shape bythe reflected light from the photoresist-applied substrate, the patternedge roughness is enhanced, and the like.

[0033] Thus, the use of the commercial g-line and i-line photoresists,as they are, which match to the pattern formation methods such as thereduction-projection exposure method in which imaging is made by meansof lenses and patterns are formed on photoresist films having a filmthickness of the order of 1 μm, etc., results in a low “efficiency forlight utilization” in the near-field exposure. As mentioned above, sincethe light passes through the microaperture of a probe resulting in asignificantly reduced light transmittance, and furthermore only about 1%of the transmitted light contributes to the exposure, a ratio of thelight contributing to the exposure to the incident light is very small.Furthermore, there is a fault that there occurs such fierce pattern edgeroughness that gives adverse results in device fabrication.

[0034] In particular, in the present specification, the degree ofcontribution to exposure of the near-field light generated by themicroaperture, that is, the degree in which the exposure making thephotosensitive compound in the photoresist cause the photochemicalreaction is referred to as the degree of the “efficiency for lightutilization” of the exposure light.

SUMMARY OF THE INVENTION

[0035] The object of the present invention is to provide a positivephotoresist which can solve the above-mentioned problems occurring inthe prior art, can achieve a high light utilization efficiency even witha small layer thickness of the photoresist layer for image formation,and can diminish the pattern edge roughness through improving thepattern rectangularity by reducing the reflection from the substrate,and to provide a photolithography method wherein the photoresist layerfor image formation formed of said positive photoresist is exposed bymeans of the near-field exposure.

[0036] The present invention which solves the above-mentioned problemsis identified by the following items.

[0037] (1) A photoresist comprising an alkali-soluble novolak resin anda photosensitive compound having a naphthoquinone diazide group whereinthe film thickness of the photoresist at the time of exposure is notlarger than 100 nm, and the absorption coefficient α (μm⁻¹) of thephotoresist for the exposure light is:

0.5≦α≦7

[0038] (2) The photoresist as set forth in (1) wherein 30 to 150 partsby weight of the photosensitive compound per 100 parts by weight of thealkali-soluble novolak resin is contained.

[0039] (3) The photoresist as set forth in (1) wherein the averagenumber of a photosensitive group in a molecule of the photosensitivecompound is not smaller than 3.

[0040] (4) The photoresist as set forth in (1) to (3) wherein the GPC(gel permeation chromatography) chart pattern area for the components ofthe alkali-soluble novolak resin having a molecular weight not smallerthan 100,000 as reduced to the standard polystyrene is less than 1% ofthe total area.

[0041] (5) The photoresist as set forth in (4) wherein theweight-average molecular weight of the alkali-soluble novolak resinfalls in the range of from 500 to 20,000.

[0042] (6) The photoresist as set forth in (4) and (5) wherein themolecular weight distribution (Mw/Mn) of the alkali-soluble novolakresin is not larger than 3.

[0043] (7) The photoresist as set forth in (1) to (6) wherein thephotoresist contains Si.

[0044] (8) A photolithography method wherein the photolithography methodincludes a step of forming a photoresist layer for image formation byuse of the photoresists as set forth in any one of (1) to (7), exposingthe formed photoresist layer by means of near-field exposure, andtransferring the pattern formed on the photoresist layer onto thesubstrate.

[0045] (9) The photolithography method as set forth in (8) wherein abuffer layer is arranged between the substrate to be processed and thephotoresist layer for image formation.

[0046] (10) A method for producing the photoresist as set forth in (1)which method comprises:

[0047] either a process for adjusting the amount of the photosensitivecompound in the alkali-soluble novolak resin; or

[0048] a process for adjusting the average number of the photosensitivegroups in a molecule of the photosensitive compound; or both thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0049]FIGS. 1A, 1B, 1C and 1D are the block diagrams outlining thephotolithography method described in Example 1 of the present invention;

[0050]FIGS. 2A, 2B, 2C and 2D are block diagrams schematically showingthe photolithography method described in Examples 2 and 5 of the presentinvention;

[0051]FIGS. 3A, 3B, 3C, 3D and 3E are the block diagrams schematicallyshowing the photolithography method described in Example 3 of thepresent invention;

[0052]FIGS. 4A and 4B show the reacted photoresist portions for the casewhere the photoresist film is thick;

[0053]FIGS. 5A, 5B, 5C and 5D are the block diagrams schematicallyshowing the photolithography method described in Example 4 of thepresent invention;

[0054]FIG. 6 represents the rectangularity; and

[0055]FIGS. 7A, 7B, 7C, 7D and 7E are the block diagrams schematicallyshowing the photolithography method described in Example 6 of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0056] The present invention will be described in detail below.

[0057] The alkali-soluble novolak resin used in the present inventioncan be prepared by condensing a phenol with an aldehyde in the presenceof an acid catalyst to synthesize an alkali-soluble novolak resin,dissolving the thus synthesized alkali-soluble novolak resin in a polarsolvent, for example, an alcohol such as methanol, ethanol, etc., aketone such as acetone, methylethyl ketone, etc., a cyclic ether such asdioxane, tetrahydrofuran, etc., and the like, and then putting it eitherin a water-polar solvent mixture system or in a nonpolar solvent such aspentane, hexane, etc., to precipitate the resin portion. In addition,alternatively, the condensation of a phenol with an aldehyde can becontrolled to prepare the alkali-soluble novolak resin, by adding thealdehyde either batchwise or continuously when the phenol is reactedwith the aldehyde.

[0058] As the phenols used in the synthesis of the alkali-solublenovolak resin employed in the present invention, there can be listedphenol, m-cresol, p-cresol, o-cresol, xylenols such as 2,5-xylenol and3,5-xylenol, m-ethylphenol, p-ethylphenol, o-ethylphenol,2,3,5-trimethylphenol, butylphenol, hydroquinone,dihydroxydiphenylpropane trimethylphenol, propylphenol,dihydroxybenzene, etc. The phenols can be used independently or as amixture of more than one thereof.

[0059] As the specific examples of the aldehydes used in the synthesisof the alkali-soluble novolak resin employed in the present invention,there can be listed formaldehyde, paraformaldehyde, acetaldehyde,benzaldehyde, phenylacetoaldehyde, furfural, etc.

[0060] As the acid catalysts used in the synthesis of the alkali-solublenovolak resin employed in the present invention, there can be listedhydrochloric acid, sulfuric acid, formic acid, oxalic acid, etc.

[0061] The above-mentioned aldehydes can be used in the quantity rangeof 0.7 to 3 moles per 1 mole of the phenol, depending on the reactionconditions. The used quantities of the above-mentioned acid catalysts ingeneral are in the range of 1×10⁻⁴ to 5×10⁻³ mole per 1 mole of thephenol, and the reaction temperature is 10 to 200° C., or preferably 70to 130° C.

[0062] The alkali-soluble novolak resin employed in the presentinvention may contain a variety of substituents, as far as the alkalisolubility is not derogated.

[0063] The photosensitive compounds employed in the present inventionare those compounds which can be synthesized by the esterificationreaction of polyhydroxy compounds with quinone diazide sulfonic acids asphotosensitive groups.

[0064] As the polyhydroxy compounds, there can be used2,3,4-trihydroxybenzophenone, 2,3,4′-trihydroxybenzophenone,2,4,6-trihydroxybenzophenone, 2,3,4,4′-tetrahydroxybenzophenone, etc.

[0065] As the quinone diazide sulfonic acids, there are listedbenzoquinone-1,2-diazide-4-phosphonic acid andnaphthoquinone-1,2-diazidesulfonic acids such asnaphthoquinone-1,2-diazide-4-sulfonic acid andnaphthoquinone-1,2-diazide-5-sulfonic acid.

[0066] The rates of the esterifications of the polyhydroxy compoundswith the quinone diazide sulfonic acids can be controlled by their molarmixing ratios. The esterification rates affect the average numbers ofthe photosensitive groups in a molecule of the photosensitive compounds,and hence the desirable average numbers of the photosensitive groups canbe obtained by adjusting the molar mixing ratios.

[0067] To be more precise, depending on how many sites reacting with thequinone diazide sulfonic acid are in the polyhydroxy compound, theaverage number of the photosensitive groups in a photosensitive compoundmolecule is not more than and close to 1 (less than 1 due to theunreacted reactants and other products) by reacting 1 mole of a quinonediazide sulfonic acid with 1 mole of a polyhydroxy compound. Similarly,by reacting 2 moles of a quinone diazide sulfonic acid with 1 mole of apolyhydroxy compound, the average number of the photosensitive groups isnot more than and close to 2, while by reacting 3 moles of a quinonediazide sulfonic acid, the average number of the photosensitive groupsis not more than and close to 3 if the number of the reaction sites isequal to or larger than 3.

[0068] The photoresist of the present invention is prepared bydissolving in a solvent the alkali-soluble novolak polymer and thephotosensitive compound prepared by the above-mentioned methods. Theamount of the photosensitive component is so adjusted that theabsorption coefficient α (μm⁻¹) of the photoresist for the exposurelight satisfies the relation:

0.5≦α≦7,

[0069] or more preferably satisfies the relation:

1≦α≦5

[0070] When the absorption coefficient α is less than 0.5 (This valuecorresponds to the 95% transmittance in the case of the 100 nm filmthickness of the applied photoresist), the transmitted amount of lightfor the exposure light with the film thickness of the formed photoresistlayer not larger than 100 nm becomes large, and the reflection from thesubstrate is increased to cause the deterioration of the rectangularityand the occurrence of the line edge roughness.

[0071] When the absorption coefficient is larger than 7 (This valuecorresponds to the 50% transmission in the case of the 100 nm filmthickness of the applied photoresist), the degree of absorption in thefilm thickness direction is increased, and hence the patterns are formedby exposure in the upper and middle portions of the photoresist layer,but it takes a long time for exposure to develop the patterns to thebottom portion of the photoresist, presumably resulting in thethroughput lowering.

[0072] As a method for preparing a photoresist in which the absorptioncoefficient α (μm⁻¹) satisfies the relation, 0.5≦α≦7, there are a methodin which the amount of the photosensitive groups in a photosensitivecompound molecule is adjusted, and a method in which the amount of thephotosensitive compound in the photoresist is adjusted, while the amountof the photosensitive groups in a photosensitive compound molecule has alimit concerning the increase in the amount of the photosensitivegroups. For instance, when a hydroxybenzophenone is used as a skeletonof the photosensitive compound, the conditions for the synthesis of ahexa compound having 6 sites reactive with the photosensitive group areso severe that the synthesis cost is increased and the throughput islowered. As for the synthesis of the fully esterified compound in whichall the photosensitive groups are esterified for all the 6 reactivesites of hexahydroxybenzophenone, the synthesis conditions arefurthermore severe, resulting in much more cost rising and throughputlowering.

[0073] With increasing amount of the photosensitive compound in thephotoresist, the photosensitive compound tends to be separated out, sothat the photoresist tends to be inappropriate for preservation withincreasing probability.

[0074] By making the absorption coefficient a to satisfy the relation,0.5≦α≦7, the transmittance can be made smaller than the case where thephotoresist is used, as it is, with the film thickness not larger than100 nm, which photoresist is prepared by assuming the prescribed filmthickness of the order of 1 μm to several 100 nm. Where thetransmittance is small, the light utilization efficiency is raised owingto the fact that the light amount of the light which entirely or partlycontributes to the photochemical reaction of the photosensitive compoundis increased, and the pattern-shape rectangularity is improved and thepattern edge roughness is reduced owing to the reduced reflection fromthe substrate. Furthermore, the difference between the exposed andunexposed portions in the photoresist in rates of dissolution in thealkaline developing solution is increased to yield the enhancedcontrast, and accordingly there is obtained the effect that therectangularity of the sectional pattern shape of the photoresist isimproved.

[0075] In order to adjust the photoresist absorptivity to theabove-mentioned range, the amount of the photosensitive compound may bemade to fall within the range of 30 to 150 parts by weight per 100 partsby weight of the alkali-soluble novolak resin.

[0076] With the less than 30 parts by weight of the photosensitivecompound, the transmittance of the exposure light is too high to yieldthe advantage that the rectangularity, resolution, and light utilizationefficiency are improved as compared to the conventional photoresists.The use of the more than 150 parts by weight of the photosensitivecompound is not realistic in terms of throughput, since with such highparts by weight no further improvement of the above-mentioned advantageis achieved, and it takes a very long exposure time to decompose thephotosensitive compound thoroughly with no remaining residuals.

[0077] Further, the transmittance of the photoresist may be adjusted tothe above values by making the average number of the photosensitivegroups in a molecule of the photosensitive compound not to be smallerthan 3.

[0078] Although it is sufficient in the above method for the absorptioncoefficient to satisfy the condition that it is not smaller than 0.5 andnot larger than 7, it is preferable for the average number of thephotosensitive groups in a molecule of the photosensitive compound tofall in the range of 3 to 5, and furthermore for the amount of thephotosensitive compound to fall in the range of 50 to 130 parts byweight per 100 parts by weight of the novolak resin, in view of thecost, throughput, and preservability, depending on the skeletalstructure of the photosensitive compound, the photosensitive group, andthe combinations thereof.

[0079] The magnitude of the pattern edge roughness acceptable infabrication of devices is said to be 10% of the pattern width. Since acause for the pattern edge roughness is assumed to be the grain diameterof the photoresist aggregate, the grain diameter of the photoresistaggregate is required to be not larger than 5 nm, on the basis of theassumed minimum pattern width of 50 nm provided by the near-fieldexposure.

[0080] Factors which determine the grain diameter of the photoresistaggregate are considered to be the molecular weight of the novolak resinused as the base polymer of the photoresist, and entanglement of thebase polymers. Thus, it is preferable for the alkali-soluble novolakresin not to have components larger than 100,000 in molecular weight.Where the polymer components larger than 100,000 in molecular weight arepresent, the grain diameter of the photoresist aggregate which isconsidered to be a cause for the pattern edge roughness is increased,and the surface quality also tends to be deteriorated.

[0081] Furthermore, it is preferable to make the weight-averagemolecular weight (Mw) of the novolak resin fall in the range of 500 to20,000. The molecular weight smaller than 500 leads to the poorfilm-forming performance, while the molecular weight larger than 20,000leads to the enhanced probability of increasing the grain diameter ofthe photoresist aggregate which is considered to be a cause for thepattern side wall shape disturbance and large pattern edge roughnessthat give rise to troubles in device fabrication.

[0082] Furthermore, the molecular weight distribution (Mw/Mn; Mn is thenumber average molecular weight) is preferably not larger than 3. Thedistribution smaller than 3 reduces the nonuniformity in developmentwhich is caused by the non-uniform dissolution rates of the photoresistpolymers in the developing solution, which is assumed to be a cause forthe pattern edge roughness.

[0083] Incidentally, the weight-average molecular weight (Mw) andmolecular weight distribution (Mw/Mn) of the alkali-soluble novolakresin can be measured with the gel permeation chromatography (GPC)method using the monodisperse polystyrene as the standard andtetrahydrofuran as solvent with the column temperature of 40° C.

[0084] With thick photoresist films, it is impossible to fabricatemicropatterns even using the near-field light, as mentioned above. Thus,for fabrication of micropatterns, the thickness of the photoresist filmis required to be small.

[0085] As for the thickness of the photoresist film, preferably it isabout the minimum aperture used in exposure. In the case of the patternformation based on the near-field exposure, the thickness of thephotoresist film is set to be not larger than 100 nm because of aimingat the micropattern formation.

[0086] When the thickness of the photoresist film set to be about theminimum aperture as mentioned above is too small to provide a sufficienttolerance to dry etching in processing the substrate being processed,the substrate to be processed can be processed by using a multilayerphotoresist method wherein a buffer layer is provided between thesubstrate to be processed and the photoresist substrate for imageformation.

[0087] As a photoresist for image formation, the photoresist of thepresent invention may contain Si (silicon atoms). The Si incorporationinto the photoresist for image formation makes it possible to reduce onebuffer layer when the multilayer photoresist method is used, and hencethe process is made convenient so that the throughput is expected to beimproved.

[0088] The addition of Si-containing compounds can be used as a methodfor incorporating Si. The Si content is generally 1 to 50 parts byweight per 100 parts by weight of the alkali-soluble novolak resin, andis preferably 15 to 30 parts by weight. With the Si content less than 1part by weight, no improvement in the dry etching tolerance is expected,as compared to the case of the null Si content. With the Si content morethan 50 parts by weight, there occur the reduced exposure sensitivity,the prevented uniform film formation, and the like.

[0089] The positive photoresist for the near-field exposure of thepresent invention, if desired, may contain the alkali-soluble resinsother than the alkali-soluble novolak resin, and such additives wellknown in the art as sensitizer, surfactants, dyes, auxiliary adhesives,preservation stabilizers, antifoaming agents, etc.

[0090] Granted that any coating method is acceptable as far as it canachieve the desired layer thickness, uniformity, etc., the applicationby use of a spin coater is preferred from its versatility.

[0091] The layer thickness of the photoresist for image formation can beadjusted, when a spin coater is used, by varying the number ofrevolutions and the revolution time of the spin coater, and thephotoresist viscosity.

[0092] When a thin layer is desired, the number of revolutions isincreased, the revolution time is extended, and the photoresistviscosity is reduced. Incidentally, the spin coating is preferablyperformed in a sealed chamber to avoid the case where the preparation ofa thin photoresist layer for image formation is forbidden by instantevaporation of the solvent.

[0093] By increasing the number of revolutions and extending therevolution time of the spin coater, even with the same photoresistviscosity, the application thickness can be made smaller to some extent.However there is a mechanical limitation in increasing the number ofrevolutions. As far as an all-purpose spin coater is used, withincreasing revolution time the film thickness of the photoresisteventually reaches a plateau, and the throughput is reduced. Thus, it isalso preferable to adjust the photoresist viscosity in order to providea thin photoresist film.

[0094] The viscosity of the photoresist of the present invention can beadjusted by adding solvents, which viscosity preferably falls in therange of 1 to 10 cP. A large amount of solvent is necessary to make thephotoresist viscosity amount to lower than 1 cP, which is uneconomical.The photoresist viscosity higher 10 cP requires a heavy mechanical loadto the spin coater or a long application time, unpreferably resulting ina reduced throughput.

[0095] As the solvent to be used in the viscosity adjustment methodbased on solvent addition, any solvent can be used which dissolves thealkali-soluble novolak resin and the photosensitive quinone diazidecompound as the photosensitive compound. From the viewpoint of safety,however, PGMEA (propylene glycol monomethyl ether acetate), ethyllactate, butyl acetate, 2-heptane, etc. are preferably usedindependently, or as mixtures thereof.

[0096] (Exposure and the Subsequent Processes)

[0097] The image-forming photoresist layer formed on the substrate to beprocessed as described above undergoes a probe-scanning exposure usingthe near-field probe and an en bloc near-field exposure with aphotomask.

[0098] In the exposure step, since the exposure-light-sensitivewavelength range of the photoresist ranges from about 200 to 500 nm, alaser with a wavelength of 200 to 500 nm or a lamp such as a mercury-arclamp, etc. operative in this range is needed to be used as an exposurelight source. There may be used such a light source as a He—Cd laserwith the wavelengths of 442 nm and 325 nm, a GaN type blue semiconductorlaser with the wavelengths near 410 nm, or an infrared laser in thesecond or third haromic generation (SHG or THG) mode. In particular, theuse of a blue semiconductor laser recently put into practical use has aneffect to make the apparatus extraordinarily compact.

[0099] The development is made subsequently to the exposure step. Thedevelopment can be performed according to the methods well known in theart. The substrate to be processed provided with the photoresistmicropatterns thus fabricated can be processed by etching, metal vapordeposition, lift off, etc., to yield a finished product.

[0100] The following comparative examples and examples will illustratethe present invention in detail. As illustrated in the following,concerning the fabrication of the micropatterns by means of thenear-field exposure, the present invention can provide a positivephotoresist for the near-field exposure wherein the efficiency for lightutilization for exposure light is high, the rectangularity in thepattern sectional shape is excellent, and the pattern edge roughness isreduced. Moreover, the present invention can provide a photolithographymethod, inclusive of the exposure process based on the near-fieldexposure, wherein the efficiency for light utilization is high, thepattern rectangularity is excellent and the pattern edge roughness isreduced.

COMPARATIVE EXAMPLE 1

[0101] A photoresist solvent PGMEA was added to the commercialphotoresist for semiconductor production containing 20 parts by weightof a photosensitive compound (naphthoquinone diazide type compound) per100 parts by weight of the alkali-soluble novolak resin as photoresistbase resin, to obtain a photoresist having a viscosity of 5 cP.

[0102] The photoresist was applied to the surface of a glass substratesubjected to the surface treatment by coating with HMDS(hexamethyldisilazane) so as to obtain a photoresist film of 100 nm inthickness, and the transmittance of the glass substrate was measured andfound to be 99%, corresponding to the absorption coefficient of 0.08.

[0103] A substrate is spin-coated with the above-mentioned photoresistin a sealed space. A glass substrate 101 having a deposited Cr layer 102of 30 nm in thickness was used as the substrate (see FIG. 1A). Under thespin-coating condition of 7,000 rpm×60 sec, the film thickness of theobtained photoresist 103 was about 50 nm.

[0104] The photoresist coated substrate underwent exposure by use of aprobe-scanning near-field exposure apparatus (FIG. 1B). By using a probe104 of 50 nm in tip aperture diameter and making an incident light of430 nm in wavelength stream into the probe, the photoresist patternshaving the widths of the order of 100 nm (FIG. 1C) was formed at thenear-field 105 formed at the probe tip.

[0105] For the photoresist patterns, the rectangularity was measured andfound to be as poor as 50%, and the pattern widths were measured to havea not smaller than 50 nm maximum-minimum difference which is 50% or moreof the pattern width to result in a large pattern edge roughness.

EXAMPLE 1

[0106] The diazide compound containing a photosensitive compound1,2-naphthoquinone diazide-5-sulfonic acid ester as the main componentin an amount corresponding to 80 parts by weight per 100 parts by weightof a photoresist base resin (alkali-soluble novolak resin) and PGMEA assolvent were added to a commercial positive photoresist forsemiconductor production containing 30 parts by weight of aphotosensitive compound (naphthoquinone diazide) per 100 parts by weightof the photoresist base resin, and the resultant was mixed to prepare apositive photoresist for near-field exposure having a viscosity of 5 cP.The absorption coefficient of the photoresist was measured and found tobe 1.0.

[0107] A substrate was spin-coated with the positive photoresist fornear-field exposure in a sealed space to form a photoresist layer forimage formation.

[0108] A glass substrate 101 having a deposited Cr layer 102 of 30 nm inthickness was used as the substrate (see FIG. 1A). Then the Cr layer wasspin-coated with the above-mentioned photoresist under the spin-coatingcondition of 7,000 rpm×60 sec. The film thickness of the photoresistlayer 103 for image formation was measured by using a film thicknessmeasurement apparatus (α-STEP500, Tencor Corp.) and found to be about 50nm.

[0109] The photoresist layer 103 for image formation underwent exposureby use of a probe-scanning near-field exposure apparatus (FIG. 1B).Using a probe 104 of 50 nm in tip aperture diameter and making anincident light of 430 nm in wavelength stream into the probe, theexposure was made with the near-field 105 formed at the probe tip. Thedevelopment was performed with an alkaline developing solution to formthe photoresist patterns having a minimum pattern width of 50 nm (FIG.1C). The formed photoresist patterns were observed with an AFM (atomicforce microscope) and a SEM (scanning electron microscope) and thepattern edge roughness was found to be not larger than 10% of theminimum pattern line width; with the absorbance raised to 1.0, therectangularity was improved and the pattern edge roughness was reduced.

[0110] Although an amount of the photosensitive compound was increased,with the same probe scanning rate as in Comparative Example 1, thephotoresist patterns having a minimum pattern size of 50 nm and a smallpattern edge roughness could be formed. This may be ascribable to theincreased fraction of the incident light contribution to the reaction ofthe photosensitive compound, and hence the light utilization efficiencycan be said to be improved.

[0111] With the formed photoresist patterns as the master patterns, wetetching was made on the Cr layer to transfer the patterns with theminimum line width of 50 nm (FIG. 1D).

EXAMPLE 2

[0112] The diazide compound containing a photosensitive compound1,2-naphthoquinone diazide-5-sulfonic acid ester as the main componentin an amount corresponding to 80 parts by weight per 100 parts by weightof the alkali-soluble novolak resin as a base resin (molecular weightdistribution (Mw/Mn) is 2.5) was added to a photoresist containing 30parts by weight of a photosensitive compound (naphthoquinone diazide)per 100 parts by weight of the base resin, and further GPMEA as solventwas added thereto, and the resultant was mixed to prepare a positivephotoresist for near-field exposure having a viscosity of 5 cP. Theabsorption coefficient of the photoresist was measured and found to be1.0.

[0113] A substrate was spin-coated with the positive photoresist fornear-field exposure under the spin-coating condition 7,000 rpm×60 sec ina sealed space to form a photoresist layer for image formation. An SOI(Silicon On Insulator) substrate 201 having an upper Si layer 202 of 50nm in thickness was used as the substrate. The film thickness of thephotoresist layer 203 for image formation was measured by using a filmthickness measurement apparatus (α-STEP500, Tencor Corp.) and found tobe about 50 nm (FIG. 2A).

[0114] The photoresist layer 203 for image formation was exposed bynear-field light from a photomask using an en bloc exposure apparatus(FIG. 2A). The photomask used consisted of the microapertures and lightshielding films 205 formed by the FIB (Focused Ion Beam) fabrication ina Cr layer deposited on a mask base 204 comprising a SiN thin filmsupported with a support 206. While the photomask was allover set closeto the surface of the photoresist layer for image formation on thesubstrate, the exposure was performed by making the incident light 207from a Hg lamp stream onto the surface (FIG. 2B), and the photoresistpatterns with the minimum line width of 50 nm could be formed bydevelopment treatment using an alkaline developing solution (FIG. 2C).The formed patterns were observed with an AFM and a SEM, and therectangularity was found to be 80% and the pattern edge roughness wasfound to be not larger than 10% of the minimum pattern line width. Sincethe pattern edge roughness defined in the present specification is suchthat it is determined by the maximum and minimum line widths of theformed patterns, the roughness numerical values does not immediatelyrepresent the relevant distribution, the dispersive distribution of thepattern widths is contracted as compared with Example 1. This may beascribed to the molecular weight distribution of the novolak resinadjusted to 2.5.

[0115] With the formed photoresist patterns as the master patterns, dryetching was made with SF₆ gas on the upper Si layer 202 to transfer thepattern having a minimum line width of 50 nm onto the Si layer 202 onthe insulating film (FIG. 2D).

EXAMPLE 3

[0116] The diazide compound containing a photosensitive compound1,2-naphthoquinone diazide-5-sulfonic acid ester as the main componentin an amount corresponding to 80 parts by weight per 100 parts by weightof a base resin (alkali-soluble novolak resin) was added to a commercialSi-containing photoresist for semiconductor production including 15parts by weight of a photosensitive compound (naphthoquinone diazide)per 100 parts by weight of the base resin and further GPMEA as solventwas added thereto, and the resultant was mixed to prepare a positivephotoresist for near-field exposure having a viscosity of 5 cP. Theabsorption coefficient of the photoresist was measured and found to be1.0.

[0117] An SOI (Silicon On Insulator) substrate 303 having an upper Silayer of 300 nm in thickness was used as the substrate. In order toapply the multilayer photoresist method, a commercial positivephotoresist was applied to the upper Si layer by using a spin coater,and then the substrate was hard-baked at 200° C. for 30 min, to form abuffer layer (hereafter, described as a thick-film photoresist as thecase may be) 301 of 0.5 μm in thickness.

[0118] Furthermore, the buffer layer was spin-coated thereon to form aphotoresist layer 302 for image formation, under the spin-coatingcondition of 7,000 rpm×60 sec. The layer thickness of the photoresistlayer 302 for image formation was measured by using a film thicknessmeasurement apparatus (α-STEP500, Tencor Corp.) and found to be about 30nm (FIG. 3A).

[0119] The photoresist layer 302 for image formation was exposed bynear-field light from a photomask using an en bloc exposure apparatus.The photomask used consisted of the microapertures and light shieldingfilms 205 formed by the FIB fabrication in a Cr layer deposited on amask base 204 comprising a SiN thin film supported with a support 206.While the photomask was allover set close to the surface of thephotoresist layer for image formation on the substrate, the exposure wasperformed by making the incident light 207 from a Hg lamp stream ontothe surface (FIG. 3B), and the photoresist pattern with the minimumpattern line width of 50 nm could be formed by development treatmentusing an alkaline developing solution (FIG. 3C). The formed pattern wasobserved with an AFM and a SEM, and the pattern edge roughness was foundto be not larger than 10% of the minimum pattern line width.

[0120] With the formed photoresist patterns as the master patterns, dryetching was made with O₂ gas to transfer the upper most layerphotoresist patterns onto the thick-film photoresist (FIG. 3D). With thethus formed thick-film patterns as the master patterns, dry etching wasmade with SF₆ gas on the upper Si layer to transfer the patterns havinga minimum pattern line width of 50 nm onto the upper Si layer on theinsulating film (FIG. 3E).

[0121] Since the increased absorption coefficient of the Si containingphotoresist made it possible to transfer the micropatterns to thethick-film photoresist, the substrate fabrication process tolerancecould be extended, so that the Si on the insulator could be processed.

EXAMPLE 4

[0122] A mixture of m-cresol and p-cresol in molar ratio 4:6 wasprepared, formalin was added to this mixture, and the condensationpolymerization was carried out according to the usual condensationmethod using the oxalic acid catalyst, wherein a novolak resin wasobtained.

[0123] On the other hand, 2,3,4,4′-tetrahydroxybenzophenone and1,2-naphthoquinone diazide-5-sulfonyl chloride in the amountcorresponding to 90 mole % of the —OH groups of the benzophenone weredissolved in dioxane, and triethyl amine was added to this solution andesterification was performed to yield a photosensitive compound. Theaverage number of the photosensitive groups of the photosensitivecompound falls in the range of 3 to 4 per a molecule.

[0124] 100 parts by weight of the novolak resin and 30 parts by weightof the photosensitive compound were dissolved and mixed in PGMEA assolvent, and then the solution was filtered with a Teflon (trade name)filter of 0.1 μm. PGMEA was added to the filtrate to adjust theviscosity of the filtrate to be 5 cP, to prepare a positive photoresistfor the near-field exposure. A film of 100 nm in thickness prepared byapplying the thus prepared photoresist exhibited a 90% transmittance forthe exposure light, corresponding to an absorption coefficient of 1.1.

[0125] Then, a photoresist layer for image formation was formed byspin-coating the substrate with the photoresist in a sealed space, underthe spin-coating condition 7,000 rpm×60 sec. An SOI (Silicon OnInsulator) substrate 201 having an upper Si layer 202 of 50 nm inthickness was used as the substrate (FIG. 5A). The layer thickness ofthe photoresist layer 103 for image formation was measured by using afilm thickness measurement apparatus (α-STEP500, Tencor Corp.) and foundto be about 50 nm.

[0126] The photoresist layer 103 for image formation underwent exposureby use of a probe-scanning type near-field exposure apparatus (FIG. 5B).Using a probe 104 of 50 nm in tip aperture diameter and making anincident light of 430 nm in wavelength stream into the probe, theexposure was made with the near-field 105 formed at the probe tip. Thedevelopment was performed with an alkaline developing solution to formthe photoresist patterns having a minimum pattern width of 50 nm (FIG.5C). The formed photoresist patterns were observed with an AFM (atomicforce microscope) and a SEM (scanning electron microscope) and thepattern edge roughness was found to be not larger than 10% of theminimum pattern line width, and the rectangularity was found to be 80%in the sectional shape, achieving an improvement as compared withComparative Example 1.

[0127] Consequently, there were formed the photoresist patterns in whichthe minimum pattern size was 50 nm and the pattern edge roughness wassmall. This may be ascribable to the reduction of the reflection fromthe Si layer 202 caused by the increased average number of thephotosensitive groups in a molecule of the photosensitive compound,eventually resulting in the reduced pattern edge roughness. Since thetransmittance was decreased as compared with the case where a commercialphotoresist was employed for the near-field exposure, it can be saidthat the incident light amount contributing to the reaction of thephotosensitive compound was increased, and hence the light utilizationefficiency was improved. It also can be said that the difference betweenthe exposed and unexposed portions in solubility to the developingsolution was increased due to the increased number of the photosensitivegroups, leading to the improved rectangularity of the pattern sectionalshape.

[0128] With the formed photoresist patterns as the master patterns, wetetching was made on the Si layer 202 to transfer the pattern with theminimum line width of 50 nm (FIG. 5D).

EXAMPLE 5

[0129] A mixture of m-cresol and p-cresol in molar ratio 4:6 wasprepared, formalin was added to this mixture, and the condensationpolymerization was carried out according to the usual condensationmethod using the oxalic acid catalyst, whereby a novolak resin wasobtained. The novolak resin was subjected to the fractionationtreatment, whereby the low molecular weight portion was discarded toprepare the novolak resin having the weight-average molecular weight of3,500.

[0130] On the other hand, 2,3,4,4′-tetrahydroxybenzophenone and1,2-naphthoquinone diazide-5-sulfonyl chloride in the amountcorresponding to 90 mole % of the —OH groups of the benzophenone weredissolved in dioxane, and triethyl amine was added to this solution andesterification was performed to yield a photosensitive compound. Theaverage number of the photosensitive groups of the photosensitivecompound falls in the range of 3 to 4 per a molecule.

[0131] 100 parts by weight of the novolak resin and 30 parts by weightof the photosensitive component were dissolved and mixed in PGMEA assolvent, and then the solution was filtered with a Teflon (trade name)filter of 0.1 μm. PGMEA was added to the filtrate to adjust theviscosity of the filtrate to be 5 cP, to prepare a positive photoresistfor the near-field exposure. The absorption coefficient of thephotoresist was measured and found to be 1.1.

[0132] Then, a photoresist layer for image formation was formed byspin-coating the substrate with the positive photoresist for thenear-field exposure in a sealed space under the spin-coating condition7,000 rpm×60 sec. An SOI (Silicon On Insulator) substrate 201 having anupper Si layer 202 of 50 nm in thickness was used as the substrate. Thelayer thickness of the photoresist layer 203 for image formation wasmeasured by using a film thickness measurement apparatus (α-STEP500,Tencor Corp.) and found to be about 50 nm (FIG. 2A).

[0133] The photoresist layer 203 for image formation was exposed bynear-field light from the photomask using an en bloc exposure apparatus(FIG. 2A). The photomask used consisted of the microapertures and lightshielding films 205 formed by the FIB (Focused Ion Beam) fabrication ina Cr layer deposited on a mask base 204 comprising a SiN thin filmsupported with a support 206. While the photomask was allover set closeto the surface of the photoresist layer for image formation on thesubstrate, the exposure was performed by making the incident light 207from a Hg lamp stream onto the surface (FIG. 2B), and the photoresistpatterns with the minimum line width of 50 nm could be formed bydevelopment treatment using an alkaline developing solution (FIG. 2C).The formed patterns were observed with an AFM and a SEM, and the patternrectangularity was found to be 80% and the pattern edge roughness wasfound to be not larger than 8% of the minimum pattern line width.

[0134] The pattern edge roughness was reduced as compared to Example 4,owing to the average molecular weight of the novolak resin specified tobe 3,500.

[0135] With the formed photoresist patterns as the master patterns, dryetching was made with SF₆ gas on the upper Si layer to transfer thepattern having a minimum line width of 50 nm onto the Si layer on theinsulating film (FIG. 2D).

EXAMPLE 6

[0136] A mixture of m-cresol and p-cresol in molar ratio 4:6 wasprepared, formalin was added to this mixture, and the condensationpolymerization was carried out according to the usual condensationmethod using the oxalic acid catalyst, whereby a novolak resin wasobtained.

[0137] On the other hand, 2,3,4,4′-tetrahydroxybenzophenone and1,2-naphthoquinone diazide-5-sulfonyl chloride in the amountcorresponding to 90 mole % of the —OH groups of the benzophenone weredissolved in dioxane, and triethyl amine was added to this solution andesterification was performed to yield a photosensitive compound. Theaverage number of the photosensitive groups of the photosensitivecompound falls in the range of 3 to 4 per a molecule.

[0138] 100 parts by weight of the novolak resin and 30 parts by weightof the photosensitive component were dissolved and mixed in PGMEA assolvent, and then the solution was filtered with a Teflon (trade name)filter of 0.1 μm. PGMEA was added to the filtrate to adjust theviscosity of the filtrate to be 5 cP, to prepare a positive photoresistfor the near-field exposure. The absorption coefficient of thephotoresist was measured and found to be 1.1.

[0139] An SOI (Silicon On Insulator) substrate 303 having an upper Silayer of 300 nm in thickness was used as the substrate. In order toapply the multilayer photoresist method, a commercial positivephotoresist was applied to the upper Si layer by using a spin coater,and then the substrate was hard-baked at 200° C. for 30 min, to form abuffer layer (hereafter, described as a thick-film photoresist as thecase may be) 301 of 0.5 μm in thickness.

[0140] An SOG (Spin On Glass) organic-solvent solution was applied tothe buffer layer 301, and an oxidized Si thin film of 100 nm inthickness was formed by heating. This is referred to as the SOG layer304. The SOG layer 304 was spin-coated with the photoresist under thespin-coating condition 7,000 rpm×60 sec, to form a photoresist layer 302for image formation. The layer thickness of the photoresist layer 302for image formation was measured by using a film thickness measurementapparatus (α-STEP500, Tencor Corp.) and found to be about 40 nm (FIG.7A).

[0141] The photoresist layer 302 for image formation was exposed bynear-field light from the photomask using an en bloc exposure apparatus.The photomask used consisted of the microapertures and light shieldingfilms 205 formed by the FIB fabrication in a Cr layer deposited on amask base 204 comprising a SiN thin film supported with a support 206.While the photomask was allover set close to the surface of thephotoresist layer for image formation on the substrate, the exposure wasperformed by making the incident light 207 from a Hg lamp stream ontothe surface (FIG. 7B), and the photoresist patterns with the minimumpattern line width of 50 nm could be formed by development treatmentusing an alkaline developing solution (FIG. 7C). The formed patternswere observed with an AFM and a SEM, and the pattern rectangularity wasfound to be 80% and the pattern edge roughness was found to be notlarger than 10% of the minimum pattern line width.

[0142] Subsequently, using the micropatterns transferred on thephotoresist layer 302 as the photomask, the patterning of the SOG layer304 was performed by dry etching (FIG. 7D). With the thus fabricatedmicropatterns on the SOG layer 304 as the photomask, the buffer layer301 was processed by means of the oxygen RIE (FIG. 7E). According to theabove-mentioned procedures, the micropatterns having the minimum patternline width of 50 nm on the photomask was transferred onto the upper Silayer on the insulating film with distinct contrast. The high aspectratio of the micropatterns due to the photoresist on the SOI substrate303 allowed to form the micropatterns which facilitated the subsequentdevice fabrication processes.

What is claimed is:
 1. A photoresist comprising an alkali-solublenovolak resin and a photosensitive compound having a naphthoquinonediazide group, wherein the absorption coefficient α (μm⁻¹) of thephotoresist for an exposure light falls in the range of 0.5<α≦7.
 2. Thephotoresist as set forth in claim 1, wherein 30 to 150 parts by weightof the photosensitive compound per 100 parts by weight of thealkali-soluble novolak resin is contained.
 3. The photoresist as setforth in claim 1, wherein the average number of a photosensitive groupin a molecule of the photosensitive compound is not smaller than
 3. 4.The photoresist as set forth in claim 1, wherein the GPC (gel permeationchromatograph) chart pattern area for the components of thealkali-soluble novolak r e sin having a molecular weight not smallerthan 100,000 as reduced to the standard polystyrene is less than 1% ofthe total area.
 5. The photoresist as set forth in claim 4, wherein theweight-average molecular weight of the alkali-soluble novolak resinfalls in the range of from 500 to 20,000.
 6. The photoresist as setforth in claim 4, wherein the molecular weight distribution (Mw/Mn) ofthe alkali-soluble novolak resin is not larger than
 3. 7. Thephotoresist as set forth in claim 1, wherein the photoresist containsSi.
 8. A method for processing a substrate to be processed, comprisingthe steps of: forming on the substrate to be processed a photoresistlayer for image formation not larger than 100 nm in thickness using thephotoresist as set forth in claim 1; exposing the formed photoresistlayer for image formation by means of near-field exposure; forming apattern by developing the photoresist layer for image formation; andtransferring the pattern formed on the photoresist layer onto thesubstrate.
 9. The method for processing a substrate as set forth inclaim 8, which furthermore comprises the steps of: forming a bufferlayer between the substrate to be processed and the photoresist layerfor image formation; and transferring the pattern formed on thephotoresist layer for image formation onto the buffer layer.
 10. Amethod for producing the photoresist as set forth in claim 1 whichcomprises: either a step of adjusting the amount of the photosensitivecompound relative to the alkali-soluble novolak resin; or a step ofadjusting the average number of the photosensitive groups in a moleculeof the photosensitive compound; or both thereof.